Lea Michele proudly displayed the 3 Bs during her Italian vacay -- boyfriend, boat, and a whole lot of her thong-framed butt.

The "Glee" star was spotted making out with her bf Matthew Paetz ... onboard a boat off Positano this weekend.

Apparently, Matthew's gigolo "research" is paying off -- cause the "coach" is scoring big time on the open sea.

That's amoré!